The rear element is just that the rear element the F stop is set by the size of the front element of the lens. The light path after the front element or other element sizes have all to due with the corrections needed color and other correction.

Are you sure that the sharpness problem are not DOF issues? At wide open even a hundred foot out the long lenses have still a very small in focus area. Your subject will be in focus but your foreground and background will be out of focus.